天津理工大學(xué)-計(jì)算機(jī)網(wǎng)絡(luò)題庫(kù)

PARTⅠ:ChoiceB

1.Whichofthefollowingservicesdoesnotthetransportlayerprovidefortheapplicationlayer?A.In-orderdeliveryofdatasegmentsbetweenprocessesB.BesteffortdeliveryofdatasegmentsbetweencommunicatinghostsC.MultiplexinganddemultiplexingoftransportlayersegmentsD.CongestioncontrolA

2.WhatarethetwoofthemostimportantprotocolsintheInternet?

A.TCPandIP

B.TCPandUDP

C.TCPandSMTP

D.ARPandDNSC

3.TheInternetprovidestwoservicestoitsdistributedapplications:aconnectionorientedreliableserviceanda(

).

A.connectionorientedunreliableservice

B.connectionlessreliableservice

C.connectionlessunreliableservice

D.InorderdatatransportserviceD

4.Processesontwodifferentendsystemscommunicatewitheachotherbyexchanging(

)acrossthecomputernetwork.

A.packets

B.datagram

C.frames

D.messagesA

5.Thejobofdeliveringthedatainatransport-layersegmenttothecorrectsocketiscalled(

).A.demultiplexing

B.multiplexing

C.TDM

D.FDMC

6.TwoimportantreasonsthattheInternetisorganizedasahierarchyofnetworksforthepurposesofroutingare:A.LeastcostandmaximumfreecircuitavailabilityB.MessagecomplexityandspeedofconvergenceC.ScaleandadministrativeautonomyD.LinkcostchangesandlinkfailureB

7.Whichofcharactersisnotdistance-vectoralgorithm’scharacters？（

）A.iterative

B.global

C.asynchronous

D.distributedD

8.ThelengthofIPV6addressis（

）bits.

A.32

B.48

C.64

D.128C

9.ThehostcomponentofaCIDRaddressoftheforma.b.c.d/25cancontainaddressesfor:A.225hosts(minus“special”hosts)B.512hosts(minus“special”hosts)C.2(32-25)hosts(minus“special”hosts)D.25hosts(minus“special”hosts)C

10.Theprimaryfunctionoftheaddressresolutionprotocol(ARP)thatresidesinInternethostsandroutersis:A.ToprovideLANrouterfunctionsB.TotranslatebetweenLANaddressesandphysicalinterfaceaddressesC.TotranslateIPaddressestoLANaddressesD.TocalculatetheshortestpathbetweentwonodesonaLANA

11.ThePOP3

protocolrunsover____andusesport____.

A.TCP

110

B.UDP

110

C.UDP

25

D.TCP

25D

12.Whenadestinationhosttransportlayerreceivesdatafromthenetworklayer,itunambiguouslyidentifiestheappropriateprocesstopassthedatatobyusingatripletconsistingof:A.Sourceport#,destinationIPaddress,andsourceIPaddressB.Destinationport#,sourceport#,processID#C.Destinationport#,sourceport#,destinationIPaddressD.Destinationport#,sourceport#,sourceIPaddressD

13.Fromthelistbelow,selecttheitemsfoundintheTCPsegmentstructurethatarenotfoundintheUDPsegmentstructure:A.ApplicationGeneratedData

B.DestinationPort#

C.SourcePort#

D.Sequence#A

14.TheRIProutingprotocolisbasedonanalgorithmthatis:A.Basedoninformationreceivedonlyfromlink“neighbors”B.AlinkstatealgorithmC.AnOSPFalgorithmD.AcentralizedroutingalgorithmB

15.Withanexteriorroutingprotocol,whichofthefollowingissuesgenerallydominatestheroutingdecisions?A.GeographicaldistancebetweenAS’sB.PolicyC.NumberofAS’straversedD.CurrentcongestionlevelsintheAS’sA

1.Endsystemareconnectedtogetherby____.A.communicationlinks

B.applicationlayer

C.transportlayer

D.thenetworklayerC

2.Whichapplication’sNOTusingTCP?

A.SMTP

B.HTTP

C.DNS

D.AllofthemB

3.Inthepollingprotocols,themasternodepollseachofthenodesina/an____fashion.

A.random

B.appointed

C.round-robin

D.uncirculated

C

4.TheDNSprotocolrunsover____andusesport____.

A.UDP

36

B.TCP

36

C.UDP

53

D.TCP

53A

5.TCPprovidesa____servicetoitsapplicationstoeliminatethepossibilityofthesenderover-flowingthereceiver’sbuffer.

A.flow-control

B.congestioncontrol

C.reliabilitycontrol

D.dataconnectionD

6.Wecanclassifyjustaboutanymultipleaccessprotocolasbelongingtooneofthreecategories:channelpartitioningprotocols,randomaccessprotocols,and____.

A.addressresolutionprotocols

B.Dynamichostconfigurationprotocols

C.link-controlprotocols

D.taking-turnsprotocols

B

8.Themaximumtransferunit(MTU)inEthernetframestructureis()byte.A.1000

B.1500

C.800

D.2000B

9.ThesocketofUDPisidentifiedby_____and_______.

A.sourceIPaddressandsourceportnumber

B.destinationIPaddressanddestinationportnumber.

C.sourceIPaddressanddestinationportnumber.

D.destinationIPaddressandsourceIPaddress.C

10.Whichisnotplugandplayinthefollowingfouritems?A.DHCP

B.Hubs

C.Routers

D.SwitchesD

11．Whichofroutersisnotdefaultrouters?A.first-hoprouter

B.sourcerouter

C.destinationrouter

D.second-hoprouterB

13.ICMPis_____.

A.theprotocolofApplicationlayer

B.theprotocolofnetworklayerC.theprotocoloftransportlayer

D.notapartofTCP/IPprotocolsB

14.Asgeneral,wehasfollowingchannelpartitioningprotocolsexcept____.

A.TDM

B.CSMA

C.FDM

D.CDMAD

15.____ismostusedforerrorreporting.

A.UDP

B.SMTP

C.FTP

D.ICMPB

16.TheheaderofIPV6is____byte.

A.20

B.40

C.60

D.80B

17.Inthenetworklayertheseservicearehost-to-hostserviceprovidedby____.（B）A.thetransportlayertothenetworklayerB.thenetworklayertothetransportlayerC.thenetworklayertothenetworklayerD.thetransportlayertothetransportlayerA

18.Ifthereisnotenoughmemorytobufferanincomingpacket,apolicythatdropthearrivingpacketcalled____.A.drop-tail

B.packetloss

C.protocol

D.encapsulationC

19.Ineithercase,a____receivesroutingprotocolmessages,whichareusedtoconfigureitsforwardingtable.

A.server

B.host

C.router

D.ModemD

20.WhichofthefollowingfunctionsdoesnotbelongtoPPP___.A.framing

B.link-controlprotocols

C.network-controlprotocols

D.errorcorrectionB

1.WhichofthefollowingservicesdoestheInternetnetworklayerprovidefortheInternettransportlayer?A.In-orderdeliveryofdatasegmentsbetweenprocessesB.BesteffortdeliveryofdatasegmentsbetweencommunicatinghostsC.MultiplexinganddemultiplexingoftransportlayersegmentsD.CongestioncontrolD

2.ThemaintaskoftheInternet’sDomainNameSystem(DNS)isto:A.TranslateportnumberstoIPaddressesB.SpecifythestandardsforInternetdomainnamesC.ProvideanauthorityforregisteringdomainnamesD.Translatemnemonic（記憶的）namestoIPaddressesA

10.TheFTPprotocolrunsover____andusesport____.

A.TCP

21

B.TCP

80

C.UDP

20

D.TCP

110C

3.RDT3.0’sreceiverFSMissameto:a)

RDT1.0

b)RDT2.1

c)RDT2.2

d)RDT2.0B

4.TheTransmissionControlProtocol(TCP)provideswhichofthefollowingservices?a)End-to-endstationaddressingb)Applicationmultiplexingc)Internetworkroutingd)Mediumaccesscontrol(MAC)D

6.Giventhattherequestedinformationisnotavailableatanyintermediatedatabases,anon-iteratedDNSqueryfromarequestinghostwouldfollowthepath:a)Rootnameserver,localnameserver,authoritativenameserverb)Authoritativenameserver,rootnameserver,hostnameserverc)Localnameserver,rootnameserver,localnameserver,authoritativenameservere)Localnameserver,rootnameserver,authoritativenameserverA

8.lectthefouressentialsteps,brieflydescribed,forterminatingaTCPconnectionbetweenaclientandaserver,assumingthattheinitiatinghostistheclient:(1)ClientsendsTCPsegmentwithACK0andfinalsequencenumber(2)ClientsendsTCPsegmentwithFIN=1andgoesintoFIN_WAITstate(3)ServersendsTCPsegmenttoACKtheclient’sFINrequestandentersCLOSE_WAITstate(4)ServersendsTCPsegmentwithFIN=0(5)ServersendsTCPsegmentwithFIN=1(6)ClientsendsTCPsegmentwithtoACKserver’sFINandenterssecondFIN_WAITstate(7)ClientsendsTCPsegmentwithFIN=0a)

2，3，5，6

b)5，1，2，3

c)1，3，5，7

d)

2，3，4，6B

10.Whencompensatingforlinkcostchangesinthedistancevectoralgorithm,itcangenerallybesaidthat:a)Increasedcostsarepropagatedquickly,i.e.,“badnews”travelsfastb)Decreasedcostsarepropagatedrapidly,i.e.,“goodnews”travelsfastc)Decreasedcostsdonotconverged)NoneoftheaboveB

14.AsanIPdatagramtravelsfromitssourcetoitsdestination:a)thesourceIPaddressischangedateachroutertoidentifythesendingrouterb)therouterusesthedestinationIPaddresstoconsultitsroutingtablec)therouterdoesnotusetheIPaddressesinthedatagramd)thedestinationIPaddressischangedateachroutertoreflectthenexthopC

15.Fromthelistbelow,choosethebitpatternwhichcouldbeavalidgeneratorvaluefortheCRCcode(R)11010:a)1110b)011010c)100101d)10011A

16.Considersendinga1300byteIPv4datagramintoalinkthathasanMTUof500bytes:a)Threefragmentsarecreated.b)Fourfragmentsarecreated.c)Threefragmentsarecreatedwithoffsets0,5001000d)Thelastfragmentconsistsofexactly300bytesofdatafromtheoriginaldatagramC

17.SupposeoneIPv6routerwantstosendadatagramtoanotherIPv6router,butthetwoareconnectedtogetherviaaninterveningIPv4router.Ifthetworoutersusetunneling,then:a)ThesendingIPv6routercreatesanIPv4datagramandputsitinthedatafieldofanIPv6datagram.b)ThesendingIPv6routercreatesoneormoreIPv6fragments,noneofwhichislargerthanthemaximumsizeofanIPv4datagram.c)ThesendingIPv6routercreatesanIPv6datagramandputsitinthedatafieldofanIPv4datagram.d)ThesendingIPv6routercreatesanIPv6datagramandinterveningIPv4routerwillrejecttheIPv6datagramD

18.WhichofthefollowingwasanimportantconsiderationinthedesignofIPv6a)fixedlength40-byteheaderandspecifiedoptionstodecreaseprocessingtimeatIPv6nodesb)128-bitaddressestoextendtheaddressspacec)differenttypesofservice(flows)definedd)alloftheaboveD

19.Anetworkbridgetableisusedtoperformthefollowing:a)MappingMACaddressestobridgeportnumbersb)ForwardingframesdirectlytooutboundportsforMACaddressesithandlesc)Filtering(discarding)framesthatarenotdestinedforMACaddressesithandlesd)AlloftheabovePARTⅡ:True/False

(1pointsperquestion–total:20points)1.TheDNSservercanupdatetherecords.

(T)2.TheTCPconnectionisadirectvirtualpipebetweentheclient’ssocketandtheserver’sconnectionsocket.

（T）3.SMTPprotocolconnectthesender’smailserverandreceiver’smailserver

(T)4.Whereasatransport-layerprotocolprovideslogicalcommunicationbetweenprocessesrunningondifferenthosts,anetwork-layerprotocolprovideslogicalcommunicationbetweenhosts.

(T)5.UDPandTCPalsoprovideintegritycheckingbyincludingright-detectionfieldsintheirheaders.

(F)6.IftheapplicationdeveloperchoosesUDPinsteadofTCP,thentheapplicationisnotdirectlytalkingwithIP.

(F)7.Whenwedevelopanewapplication,wemustassigntheapplicationaportnumber.

(T)8.Real-tineapplications,likeInternetphoneandvideoconferencing,reactverypoorlytoTCP’scongestioncontrol.

(T)9.ThesenderknowsthatareceivedACKorNAKpacketwasgeneratedinresponsetoitsmostrecentlytransmitteddatapacket.

(T)10.Tosimplifyterminology,wheninanInternetcontext,werefertothe4-PDUasaunit.

(F)11.DValgorithmisessentiallytheonlyroutingalgorithmusedinpracticetodayintheInternet。(F)12.Everyrouterhasmanyforwardingtables.(F)13.Ineithercase,aserverreceivesroutingprotocolmessages,whichareusedtoconfigureitsforwardingtable.(F)14.AmongtheASs,theyrunthesameinter_ASroutingprotocol.

T15.Routingprotocols’jobistodeterminethepathtakebyadatagrambetweensourceanddestination.

T16.OSPFwasoneof

theearliestintra_ASInternetroutingprotocols.

F17.TheIPV6datagramsaremoresimpler,morestreamlined.

(T)18.Forwardingtablesindatagramnetworkscanbemodifiedatanytime.（T）19.InaVCnetwork,thenetwork’sroutersmustmaintainconnectionstateinformationfortheongoingconnections.（T）20.Theroutersofdatagramnetworksuseforwardingtable,theroutersofvirtual-circuitnetworksdon’tuseforwardingtable.（F）PARTⅡ:Brieflyanswerthefollowingquestions.1.Inthe“10BaseT”,Tstandsfor(

).(twistedpair)2.(

)and(

)aretwoofthemostimportantprotocolsintheinternet.

(TCP)(IP)3.Inaclient-serverarchitecture,thereisanalways-onhost,calledthe(

),whichservicesrequestsfrommanyotherhosts,called(

).

(server)(clients)4.ThebroadcastdestinationIPaddressis(

).

(55)5.Twoofthemoreimportant(

)

protocolsarepollingprotocolsandtoken-passingprotocol.

(taking-turns)6.Transport-layerprotocolsareimplementedinthe(

),butnotinnetworkrouter.(endsystem)7.Theportnumbersranging(

)arecalledwell-knownportnumbers.(0～1023)8.The(

)

algorithmisusedinmanyroutingprotocolsinpractices,includingtheInternet’sRIPandBGP,NovellIPX,andtheoriginalARPAnet.

(DV/distance-vector)9.Internetconsistsofhierarchyofinterconnected(

).

(ISPs)10.InMACaddress,broadcastaddressis(

).

(FF-FF-FF-FF-FF-FF)11.ASisacollectionof(

)underthesameadministrativeandtechnicalcontrol.

(routers)12.(

)andFDMaretwotechniquesthatcanbeusedtopartitionabroadcastchannel’sbandwidthamongallnodessharingthatchannel.

(TDM)13.Asthequeuesgrowlarge,therouter’sbufferspacewilleventuallybeexhaustedand(

)

packetlosswilloccur.

(packetloss)14.Theroleofthenetworklayeristomove(

)fromasendinghosttoareceivinghost.(packets)15.(

)referstotherouter-localactionoftransferringapacketfromaninputlinkinterfacetotheappropriateoutputlinkinterface.

(Forwarding)16.TheInternet’snetworklayerprovidesasingleservice,knownas(

).(best-effortservice)17.Theunitsofdataexchangedbyalink-layerprotocolarecalled(

).

(frames)

18.The(

)-layerprotocoldefinestheformatofthepacketsexchangedbetweenthenodesattheendsofthelink,aswellastheactionstakenbythesenodeswhenthepacketsaresentandreceived.

(link)1．(3points)Whatisthedifferencebetweenahostandendsystem?Listthetypesofendsystems.IsaWebServeranendsystem?答：Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,Internet-connectedPDAs,WebTVs,etc.1．(1points)WhyisthereaWebcache?答：減少接入網(wǎng)絡(luò)的流量、減小延遲2．(3points)PleasebrieflydescribethefunctionofthefivelayersinTCP/IParchitecture(NOTE:includetheprotocoldataunit)答：1．應(yīng)用層，提供各種網(wǎng)絡(luò)應(yīng)用，運(yùn)行網(wǎng)絡(luò)應(yīng)用程序。Message。2．運(yùn)輸層，提供端到端進(jìn)程之間的數(shù)據(jù)傳輸。Segment。3．網(wǎng)絡(luò)層，主機(jī)到主機(jī)的傳輸。Datagram4．鏈路層，相鄰節(jié)電數(shù)據(jù)傳輸，frame5．物理層，透明傳輸比特流，bitstream.3.(2points)WhataretheservicesofTCP?答：connection-orientedservice;reliable,in-orderbyte-streamdatatransferflowcontrol:congestioncontrol:4.(2points)Pleasebrieflycompare:datagramnetworkversusvirtualcircuit(VC)network.答：數(shù)據(jù)報(bào)網(wǎng)絡(luò)是面向非連接的服務(wù)，不可靠的服務(wù)，沒(méi)有流量控制和擁塞控制。虛電路網(wǎng)絡(luò)是面向連接的網(wǎng)絡(luò)，可以提供可靠的傳輸服務(wù)，具有流量控制和擁塞控制。5.(2points)Whatisthedifferencebetweenroutingandforwarding?答：Forwardingisaboutmovingapacketfromarouter’sinputlinktotheappropriateoutputlink.Routingisaboutdeterminingtheend-to-routesbetweensourcesanddestinations.6.(3points)HowbigisMACaddressspace?TheIPv4addressspace?TheIPv6addressspace?答：MAC:248，IPv4：232；IPv6：21287.(4points)Fillintheblank:

1)RIPadvertisementstypicallyannouncethenumberof(

hops

)tovariousdestinations.2)BGPupdatesannouncethe(

ASs

)tovariousdestinations

3)ForSelectiveRepeatprotocol,sequencenumberspace>=(

2

)timeswindowsize.4)TheefficiencyofSlottedALOHA=(

2

)timesbiggerthanPureALOHA.8.(2points)ConsidersendingMpacketsoverapathofNlinks.AssumethateachpackethasLbits.EachlinktransmitsatRbps.Thenetworkislightlyloadedsothattherearenoqueuingdelays.Propagationdelayandnodeprocessingdelayarenegligible.Supposethenetworkisapacket-switcheddatagramnetworkandaconnectionlessserviceisused.Nowsupposeeachpackethas2hbitsofheader.Howlongdoesittaketosendthepacket?(allpacketsgothroughthesamepath)答：(N+M-1)*(L+2h)/R9.(2points)IfallthelinksintheInternetweretoprovidethereliabledeliveryservice,wouldtheTCPreliabledeliveryserviceberedundant?WhyorWhynot?答:AlthougheachlinkguaranteesthatanIPdatagramsentoverthelinkwillbereceivedattheotherendofthelinkwithouterrors,itisnotguaranteedthatIPdatagramswillarriveattheultimatedestinationintheproperorder.WithIP,datagramsinthesameTCPconnectioncantakedifferentroutesinthenetwork,andthereforearriveoutoforder.TCPisstillneededtoprovidethereceivingendoftheapplicationthebytestreaminthecorrectorder.Also,IPcanlosepacketsduetoroutingloopsorequipmentfailures.3.(3points)3.(3points)ConsideraccessingyouremailwithPOP.Supposeyouhaveconfiguredyourpopmailclienttooperateinthedown-load-and-deletemode.Completethefollowingtransaction.C:listS:1498S:2912S:.C:retr1S:blahblah…S:…………blahS:.???答：C:dele1C:retr2S:(blahblah…S:………..blah)S:.C:dele2C:quit4.(3points)Wherecanqueuingoccurinarouter?Brieflyexplaintheconditionsthatleadtosuchqueuing.答：Queuingcanoccuratboththeinputportsandtheoutputportsofarouter.Queuingoccursattheoutputportwhenthearrivingrateofpacketstotheoutgoinglinkexceedsthelinkcapacity.Queueoccursonaninputportwhenthearrivingrateofpacketsexceedstheswitchcapacity;head-of-the-lineblockingcanalsocausequeuingattheinputports.5.(3points)DescribewhyanapplicationdevelopermightchoosetorunanapplicationoverUDPratherthanTCP.答：AnapplicationdevelopermaynotwantitsapplicationtouseTCP’scongestioncontrol,whichcanthrottletheapplication’ssendingrateattimesofcongestion.Often,designersofIPtelephonyandIPvideoconferenceapplicationschoosetoruntheirapplicationsoverUDPbecausetheywanttoavoidTCP’scongestioncontrol.Also,someapplicationsdonotneedthereliabledatatransferprovidedbyTCP.6.(3points)SupposeClientAinitiatesaTelnetsessionwithServerS.Ataboutthesametime,ClientBalsoinitiatesaTelnetsessionwithServerS.Providepossiblesourceanddestinationportnumberfora.ThesegmentssentfromAtoSb.ThesegmentssentfromBtoSc.ThesegmentssentfromStoAd.ThesegmentssentfromStoBe.IfAandBaredifferenthosts,isitpossiblethatthesourceportnumberinthesegmentsfromAtoSisthesameasthatfromBtoS?f.Howaboutiftheyarethesamehost?答：sourceportnumbersdestinationportnumbersa.A→S46723b.B→S51323c.S→A23467d.S→B23513e.yesf.no7.(4points)Fillintheblank:

1)RIPadvertisementstypicallyannouncethenumberof(

hops

)tovariousdestinations.2)BGPupdatesannouncethe(

ASs

)tovariousdestinations

3)ForSelectiveRepeatprotocol,sequencenumberspace>=(

2

)timeswindowsize.4)TheefficiencyofSlottedALOHA=(

2

)timesbiggerthanPureALOHA.8.(3points)ConsidersendingapacketoverapathofNlinks.AssumethateachpackethasLbits.EachlinktransmitsatRbps.Thenetworkislightlyloadedsothattherearenoqueuingdelays.Propagationdelayandnodeprocessingdelayarenegligible.Supposethenetworkisapacket-switcheddatagramnetworkandaconnectionlessserviceisused.Nowsupposeeachpackethas2hbitsofheader.Howlongdoesittaketosendthepacket?答：(L+2h)/R*N9.(3points)IfallthelinksintheInternetweretoprovidethereliabledeliveryservice,wouldtheTCPreliabledeliveryserviceberedundant?WhyorWhynot?答:AlthougheachlinkguaranteesthatanIPdatagramsentoverthelinkwillbereceivedattheotherendofthelinkwithouterrors,itisnotguaranteedthatIPdatagramswillarriveattheultimatedestinationintheproperorder.WithIP,datagramsinthesameTCPconnectioncantakedifferentroutesinthenetwork,andthereforearriveoutoforder.TCPisstillneededtoprovidethereceivingendoftheapplicationthebytestreaminthecorrectorder.Also,IPcanlosepacketsduetoroutingloopsorequipmentfailures.10.(Total4points)CDMASupposethatfournodesusingCDMA(CodeDivisionMultipleAccess)protocoltotransporttheirdata.Thechippingsequencesarelistedrespectivelyasbelow:A:(-1,+1,–1,+1,+1,+1,–1,-1)B:(-1,+1,–1,–1,–1,–1,+1,-1)C:(-1,–1,–1,+1,+1,–1,+1,+1)D:(-1,–1,+1,–1,+1,+1,+1,-1)(Computingprocessisneedless)SupposethataCDMAreceiverhasreceivedanencodeddataas(-1,+1,-3,+1,-1,-3,+1,+1)fromanaggregatesignalchannelinonebit-slottime,whichnode/nodeshas/havesentdata?What’sthecorrespondingdatabiteachofthemsent?(3points)Answer:

Asendnothing,Bsend1,Csends1,Dsends0.11.(2points)Inthefigurebelow,H1remainsinsameIPsubnet,brieflydescribehowtheswitchwouldknowwhichAPisassociatedwithH1whenH1ismovingwithinBSS1andBSS2.答：通過(guò)自學(xué)習(xí)的方法12.(3points)Thisquestionisaboutthe“GSM:handoffwithcommonMSC”.Accordingtothefigurebelow,orderingthe8steps.A.OldBSSinformsMSCofimpendinghandoff,provideslistof1+newBSSsB.OldBSStellsmobile:performhandofftonewBSSC.NewBSSallocatesradiochannelforusebymobileD.NewBSSsignalsMSC,oldBSS:readyE.MSC-oldBSSresourcesreleasedF.MSCsetsuppath(allocatesresources)tonewBSSG.MobilesignalsvianewBSStoMSC:handoffcomplete.

MSCreroutescallH.Mobile,newBSSsignaltoactivatenewchannelAnswer:AFCDBHGE13.(3points)WhyisanARPquerysentwithinabroadcastframe?WhyisanARPresponsesentwithinaframewithaspecificdestinationMACaddress?答：AnARPqueryissentinabroadcastframebecausethequeryinghostdoesnotwhichadapteraddresscorrespondstotheIPaddressinquestion.Fortheresponse,thesendingnodeknowstheadapteraddresstowhichtheresponseshouldbesent,sothereisnoneedtosendabroadcastframe(whichwouldhavetobeprocessedbyalltheothernodesontheLAN).2.Explainthedifferencebetweengo-back-Nandselectiverepeat.(2points)Withgo-back-N,wheneverareceiverdetectsamissingordamagedframek,allframessucceedingkareignoredandthesenderisforcedtostartretransmissionatframek.Withselectiverepeat,itispossibletoretransmitonlyframek;thereceiverbufferssuccessiveframes.3.Wherecanqueuingoccurinarouter?Brieflyexplaintheconditionsthatleadtosuchqueuing.(2points)Queuingcanoccuratboththeinputportsandtheoutputportsofarouter.Queuingoccursattheoutputportwhenthearrivingrateofpacketstotheoutgoinglinkexceedsthelinkcapacity.Queueoccursonaninputportwhenthearrivingrateofpacketsexceedstheswitchcapacity;head-of-the-lineblockingcanalsocausequeuingattheinputports.4.Satellitelinksoftenexhibithighpropagationtimesandlowtransmissionspeed.Whatdoesthismean?(2points)Itmeansthatitcantakealongtimeforthestartofasignalthatrepresentsabittoreachthesender,andthatthenumberofbitsthatcanbetransmittedpertimeunitislow.5.ThedifferencebetweenUDPandIPpacketsisminimal.Whyshouldn’tapplicationsuseIPdirectly?(2points)Itisamatterofseparatingconcerns:thetransportlayershouldofferaninterfacetoapplicationsthathidestheunderlyingnetwork.Forconnectionlessservices,itdoessobymeansofUDP.Furthermore,notethatIPitselfhasnonotionofports.Thistypeofaddressbelongstotheabstractionprovidedbythetransportlayerandofwhichapplicationsmakeheavyuse.Thereis,ingeneral,nowaythatanapplicationcangetanIPpacketsentdirectlytoanapplication.6.Cantwocomputersthathaveadifferentimplementationofthesameprotocolexchangemessages?Explainyouranswer.(2points)Yes,theprotocolprescribestheformatofmessages,andthepreciserulesformessageexchange.Youcaneasilyhavedifferentimplementationsofthesameprotocol.7.Whatarethedifferencesbetweenroutingandforwarding?Brieflyexplaineachofthem.(2points)Forwarding:movepacketsfromrouter’sinputtoappropriaterouteroutput.Routing:determineroutetakenbypacketsfromsourcetodestination.8.Brieflyexplaintheformula“l(fā)astByteSent-lastByteAcked≤min{CongWin,RcvWin}”.Ifnecessary,youcandrawafiguretodescribeyouranswer.(2points)Duetothecongestioncontrolandflowcontrol,TCPlimitstheamountofunacknowledgeddataatasendermaynotexceedtheminimumofcongwinandRcvwin.9.EthernetfollowsaCSMA/CDprotocolscheme.Explainhowthisschemeworks.(2points)CSMA/CDstandsforCarrierSenseMultipleAccesswithCollisionDetection.Thismeansthatwhenanodewantstotransmitdata,it(1)sensesthecarrieruntilnotransmissionsaredetected,(2)startstransmission,and(3)continuestocheckforacollision.Ifacollisionhappens,thenodestopsandwaitsarandomperiodbeforestartingwithstep(1)again.10.Foraslidingwindowprotocol,itisnecessarytohavethewindowsizeatmosthalfoftherangeofsequencenumbers.Why?(2points)Supposeareceiverhasjustreceivedframe#N,anditadvancesitswindowsuchthat(anew)frame#Nisallowedtobetransmitted.Iftheacknowledgementforthereceiptofframe#Nwaslost,thenthesenderwilleventuallyretransmittheoriginalframe,butwhichwillnowbeconsideredasfreshframebythereceiver.1.Explainthedifferencebetweenaconnection-orientedmessagingserviceandaconnectionlessmessagingservice.(2points)Aconnection-orientedservicerequiresthatapeerfirstsetsupaconnectiontothetargetpeerbeforedatacanbeexchanged.Inaconnectionlessservice,messagesaresentdirectlytoapeer,r